Paste x-flipped pose is all jacked up
Your .gif doesn't look like it, but if you are following the following procedure:


Select bones
Copy pose
Paste x-flipped pose


Then, you may have a whole mess of mismatched rolls, so:


Select armature and switch to edit mode
Select all bones
Press alt-R to clear roll, or select from the menu Armature/Bone Roll/Clear Roll